Apparatus for inspecting materials by wave trains



July 31, 1951 R. 8. DE LANO, JR 2,562,449

APPARATUS FOR INSPECTING MATERIALS BY WAVE TRAINS Filed Jan. 17, 1946 2 Sheets-Sheet 1 IN V EN TOR.

Patented July 31, 1951 APPARATUS FOR INSPECTING MATERIALS BY WAVE TRAINS Ralph B. De Lano, In, New York, N. Y., assignor to Sperry Products, Inc., Hoboken, N. J., a corporation of New York Application January 17, 1946, Serial No. 641,706

7 Claims.

This invention relates to a means for testing materials by means of wave trains of radio, supersonic or other high frequency signals. One such means for testing is disclosed in the patent to F. A. Firestone No. 2,280,226, granted April 21, 1942 which discloses the method of sending wave trains into an article under test and measuring the time interval which elapses between the sending of the signal and receiving of the reflection from defects within the article. The transmitted pulse may be applied to the article by a piezoelectric crystal and the reflected pulse may be received either by the same crystal or another crystal, and the output of the crystal may be impressed on the input of a receiver amplifier. Heretofore, as disclosed in the said Firestone patent, the output of the amplifier was impressed on the electrode of a cathode ray oscilloscope to vary a sweep and thus give a visual electronic in dication of the various outputs from the receiver amplifier. In my co-pending application Ser. No. 636,030 filed December 19, 1945, now Patent No. 2,507,854, I have disclosed means for cutting out the indications due to the generation of the Pulse and its reflection from the rear surface of the article so that only the reflections due to defects within the article will be indicated. In said application I accomplish the result by rendering the receiver amplifier ineffective during predetermined intervals. It is one of the objects of this invention to accomplish th same result without rendering the receiver ineffective.

It is another object of this invention to provide, in a system as described above, an indicator, such as a meter, having a mechanically movable member, instead of a cathode ray oscilloscope, said meter having means for indicating not only the presence of a defect but the distance of the defect below the entering surface of the article through which the wave trains are transmitted.

It is a further object of this invention to provide an indicator of the type described above wherein the distance of the defect below the entering surface, which is a function of time of travel of the pulse, is converted into voltage which is a linear function of distance, and utilizing the voltage to actuate an indicator.

Further objects and advantages of this invention will become apparent in the following detailed description thereof:

In the accompanying drawings,

Fig. 1 is a wiring diagram illustrating one embodiment of my invention.

Figs. 2A and 2B are diagrammaticillustrations of an article under test showing defects at dif- 2 ferent distances beneath the surface and indicating the different periods of time necessary for a wave trainto travel from the defect back to the entering surface.

Figs. 3A and 3B are diagrammatic representations of dials which may be employed in connection with the meter which indicates the distances below the surface of the various defects indicated in Figs. 2A and 2B.

Reference to Fig. 1 discloses my invention applied to a supersonic testing device wherein wave trains or pulses are periodically generated by a suitable pulse generator III. Each pulse or wave train as it is generated is impressed upon a crystal II which is in engagement with a work piece I2 which it is desired to test for defects. The pulse will cause the crystal I I to oscillate and send a train of mechanical vibrations through the work piece I2 from the front or entering surface S through the block to the opposite or rear surface S. At the same time that the pulse is impressed on the crystal II, it is also impressed on an amplifier I5 whose output may actuate an indicator I6 which, in the present invention, is a meter having a mechanically movable element starch as pointer II operating over a suitable dial As shown in Fig. 1 the pulse generator may comprise a synchronizer 30 which may take the form of a gas discharge tube 3I, which is periodically discharged by reason of the fact that a condenser 32 is periodically charged from a source of positive voltage 33. As the condenser accumulates a positive charge to a predetermined degree, sufllcient positive voltage will be placed on the plate of the gas tube 3| to cause the tube to discharge. Thus will generate 'a negative voltage which is'passed through a phase inverter tube 35 whose output will, therefore, be a positive voltage which will be placed upon the grid of a gas discharge tube 31 to generate a pulse. Thus, each time the condenser 32 discharges, tube 31 will discharge to generate a pulse having an exponential decay characteristic. Momentary frequency is determined by the condenser 38 and coil 39. This pulse is applied to crystal I I to generate the wave train which passes into the work piece I2. It is apparent that the wave train which is applied to the crystal I I will by connection ll be applied also to the input grid G of tube 50 of the receiver amplifier I5. Similarly, the reflection of the wave train from the rear surface S would, on striking the crystal II, generate a pulse which would also be applied to the amplifier Ii. Inaddition, the reflections of the wave train from defects such as D and D within the article would also be received by the crystal ii and impressed on the amplifler. The output from amplifier I is .caused to actuate the indicator it by causing a voltage to be impressed on the grid G of a gas discharge tube Oil which, upon firing, causes a condenser ll to discharge. The discharge of condenser 6i places the proper polarity across the circuit of indicator It to render a diode tube 63 conductive so that current passes through the circuit including the tube 63 and indicator l6.

Unless means were provided for inhibiting the discharge of tube 60 during the generation of the wave train and during the reception of the reflection of the wave train from the rear surface S of the article, it is apparent that the indicator It would be actuated throughout these periods and it would be difficult or impossible to differentiate the indications due to reflections of a defect from those indications due to the generation of the pulse and its reflection from the opposite surface of the article. Therefore, I provide means which will (1) block the indicator during the generation of the pulse, (2) block the indicator before the reflection of'the generated pulse from a surface S is received by the crystal, and (3) maintain the indicator blocked from a time just before the reflection of the wave train from S is received until the end of the succeeding generated pulse. Therefore, the indicator will be effective to be operated by the output from amplifier II during the interval between the end of the generated wave train and a time just before the reflection of this pulse from surface S is received.

To accomplish the hereinbefore described functions, I provide means for preventing flring of the gas tube ill during the above described blocking intervals. This means may comprise, in the circuit from condenser 81 to the plate of gas tube 80, a pentode tube III whose plate II is connected to condenser II and which has a control grid G" on which a blocking voltage is placed during the above described blocking intervals so that, even though there is an output from the amplifier II in response to the generated wave train and to its reflection from surface S, the tube III will nevertheless not fire because tube III which controls the voltage on plate 12 is not conductive during these intervals.

In order to place the blocking voltage on grid G" while the pulse is being generated and while the reflection from S is being received, there is provided a blocking puke generator III which is in the form of a multivibrator having one tube ll normally conductive and the other tube I2 normally biased to cutoff. The generation of the wave train or pulse sends a voltage through connection II by way of time constant means, comprising resistor l4 and condenser 88, to the grid 86 of tube I! which willrender the tube III conductive and thus cause a pulse to be generated which, after being amplified by tube 81, is impressed through nnection ll on the control grid G" to render th tube II conductive to place the proper plate voltage on plate 12 so that, when the output of amplifier il places a positive signal on grid G, tube ll can flre to permit condenser II to discharge and thus operate the indicator is.

In order that the tube ll shall not be rendered conductive until the generation of the wave train has been completed, the time constant elements 84, ii are so chosen that the necessary bias is not placed on grid 80 until the generation. of the pulse has ended and, therefore, there is no output from the blocking pulse generator until such pulse generation has terminated.

Once the pulse from the blocking pulse generator 80 is placed upon grid G, tube IQ is rendered conductive to place the necessary voltage on the plate of tube 60, as described, and any reflections of the wave train from defects such as D or D will cause amplifier I! to place a positive voltage on grid G to cause tube III to fire. As soon as tube 80 fires, condenser 6| begins to discharge and, since tube III is a pentode tube, this discharge will be linear with respect to time, and, therefore, distance, because the plate current of such a tube is linear. In other words, the voltage discharge of condenser BI is a direct function of time. The discharge of condenser 6| will continue until tube 10 is rendered non-conductive. This cut-off point is always the same and is chosen so that it will occur just before the reflection of the generated pulse from surface S is received by crystal H. For this purpose, the blocking pulse generator circuit is provided with time constant means in the form of. a condenser 90 and resistor 9| so that tube 82 will remain conductive for a time interval beginning when the generation of the wave train ends and continuing until just prior to the reception of the reflection of the wave train from surface S by crystal H.

Thus, it will be seen that tube I0 is conductive for the above described interval beginning when the generation of the pulse ends and ending just prior to the reception by crystal ii of the reflection of the pulse from surface S. During this interval any output from amplifier l5 must be due only to the reflection received by crystal II from a defect such as D or D. Such output of amplifier l5 will cause tube to fire and condenser H to begin'to discharge. The point in the travel of the pulse from surface S to surface S and back to surface S, at which the tube fires and condenser 6| begins to discharge, is a function of the distance of the defeet below the surface 5. Thus, the defect D lying close to surface S will begin to reflect the pulse very soon after the pulse enters the block I! and hence tube ill will flre and condenser 6| will discharge from the time the pulse reflected from D begins to be received and continuing until the pulse has travelled to S and back to S. This is a relatively long period. In contrast, a defect such as D lying close to the rear surface S will fire tube 80 and begin to discharge condenser ll only after the pulse has travelled the distance from S to D and back to S. Therefore. the condenser Iii will discharge for a relatively shorter period of time when reflected from a pulse such as D than when reflected from a pulse such as D. Since the discharge of condenser BI is always cut off at the same time. i. e., just before the reflection of the pulse from S reaches crystal I l, the time during which condenser 6| discharges is a function of the distance of the defect below the surface S. Stated diflerently,. the longer the condenser GI discharges, the closer to surface S is the defect.

vThis is shown graphically in Figs. 2A and 23 and in Figs. 3A and 3B. Thus the solid lines in Figs. 2A and 23 indicate the portion of the travel of the pulse through the article during which the condenser discharges, said time being inversely proportional to the distance of the defeet below the entering surface. The longer the condenser 6| discharges, the greater will be the change of potential across the tube 63 to give a greater indication on meter ii. The further the '5 pointer l'l moves, the closer to the entering surface S is the defect. Therefore, the dial is calibrated with diminishing figures representing distances so that the further the pointer l1 moves the smaller is the figure indicated since this flgure represents distance beneath the surface S.

By the above described arrangement, the indicator IE will give an indication not only of the presence of a defect but also of the distance of the defect beneath the entering surface of the article. It is also possible at the same time to obtain an indication of the magnitude of the defect since such magnitude is proportional to the amount of wave train reflection received at the crystal II from the respective defect. Therefore, there may be taken off the output from the amplifier l5 at, for instance, point 95, a pulse which may be impressed on a circuit similar to that in which indicator I6 is contained and comprising a condenser 96, a diode 91 and an indicator 98. The circuit of indi- :ator 98 will, therefore, become conductive if there is an output from the amplifier and the degree of movement of the pointer 99 will indicate the magnitude of the defect. In order that indicator 98 shall function only when indicator I6 is rendered effective, a blocking pulse voltage from the generator 80 is applied to the diode 91 at the same time that a blocking pulse voltage is applied to tube 10. The cathode llll is normally biased positive with respect to anode I02 but the blocking pulse voltage counteracts this bias. No appreciable current flows from the blocking pulse generator at this time because of impedance I00. However, output from the amplifier while the bias is counteracted will cause positive cycles of current to flow through diode 91 while negative cycles fiow through meter 98 of a magnitude proportional to the output of the amplifier which is proportional to the size of the defect.

Since the pulses are generated periodically at short intervals, a series of indications will be received. by indicators I6 and 98 at each position of the crystal ll so that the indications will be repeated and the indicators will in effect give an indication proportional to the peak voltage that is impressed across their input terminals.

The foregoing description of the invention is merely illustrative and changes may be made within the scope of the appended claims.

Having described my invention, what I claim and desire to secure by Letters Patent is:

1. In a device for testing material, means for generating a pulse, means for transmitting the pulse into the material through an entering surface thereof and for receiving reflections, a receiver actuated by said transmitting and receiving means, indicating means including a gas discharge tube having a plate and a grid, a circuit for said plate, means for maintaining said plate circuit ineffective until the generation of the pulse ends, and means for again rendering said plate circuit ineffective prior to the reception of the reflected pulse from the opposite surface of the material, the grid being connected to the output of the receiver whereby the gas tube will fire in response to a signal from the receiver when the plate circuit is effective.

2. In a device for testing material, means for generating a pulse, means for transmitting the pulse into the material through an entering surface thereof and for receiving reflections, a receiver actuated by said transmitting and receiving means, indicating means including a gas discharge tube having a plate and a grid, 9. circuit for said plate including a second tube having a control grid, means for placing a blocking voltage on the control grid for rendering said plate circuit and the gaseous discharge tube ineffective until the generation of the pulse ends and again placing a blocking voltage on the control grid prior to the reception of the reflected pulse from the opposite surface of the material, the grid being connected to the output oi. the receiver whereby the gas tube will fire in response to asignal from the receiver resulting from a reflection of the pulse from a defect within the material when said plate circuit is effective.

3. In a device for testing material, means for generating a pulse, means for transmitting the pulse into the material through an entering surface thereof and for receiving reflections, a receiver actuated by said transmitting and receiving means,'indicating means including a gas discharge tube having a plate, a circuit for said plate including a second tube having a plate with linear plate current characteristic and having a control grid, means for placing a blocking voltage on the control grid for rendering the plate circuit of the gaseous discharge tube ineffective until the generation of the pulse ends and again placing a blocking voltage on the control grid prior to the reception of the reflected pulse from the opposite surface of the material, the grid of the gas tube being connected to the output of the receiver whereby the gas tube will fire in response to a signal from the receiver when the plate circuit of the gas tube is rendered effective, a condenser, means for normally charging the condenser, the condenser being connected to discharge through the plate of the second tube whereby linear discharge of the condenser is obtained, the discharge of the condenser commencing when the gas tube fires and continuing until the second tube is rendered ineffective, and! means whereby said indicating means is actuated in response to discharge of said condenser.

4. In a device for testing material, means for generating a pulse, means for transmitting the pulse into the material through an entering surface thereof and for receiving reflections, a receiver actuated by said transmitting and receiving means, indicating means including a gas discharge tube having a plate, a circuit for said plate including a second tube having a plate with linear plate current characteristic and having a control grid, means for placing a blocking voltage on the control grid for rendering the plate circuit of the gaseous discharge tube inefiective until the generation of the pulse ends and again placing a blocking voltage on the control grid prior to the reception of the reflected pulse from the opposite surface of the material, the grid of the gas tube being connected to the output of the receiver whereby the gas tube will fire in response to a signal from the receiver when the plate circuit of the gas tube is rendered effective, a condenser, means for normally charging the condenser, the condenser being connected to discharge through the plate of the second tube whereby linear discharge of the condenser is obtained, the discharge of the condenser commencing when the gas tube fires and continuing until the second tube is rendered ineffective, said indicating means including a mechanically movable member,- and means whereby said member is actuated in response to the discharge of said condenser.

5. In a device for testing material, means for generating a pulse, means for transmitting the pulse into the material through an entering surface thereof and for receiving reflections, a receiver actuated by said transmitting and receiving means, indicating means adapted to be actuated by theoutput from said receiver, blocking means actuated by said pulse generating means, said blocking means including means for rendering said indicating means unresponsive to the output of said receiver until the generation of the pulse ends and a second means for rendering the indicating means again unresponsive to the output of said receiver prior to the reception of the reflected pulse from the opposite surface of the material, means actuated by the receipt Of a reflection from a defect during the interval that the indicator is responsive for setting the indicating means into operation, and means for continuing said operation of said indicating means as long as said indicating means is responsive to the output of said receiver.

6. In a device for testing material, means for generating a pulse, means for transmitting the pulse into the material through an entering surface thereof and for receiving reflections, a receiver actuated by said transmitting and receiving means, indicating means actuated by the output from said receiver, blocking means actuated by said pulse generating means, said blocking means including means for rendering said indicating means unresponsive to the output of said receiver until the generation of the pulse ends and a second means for rendering the indicating means again unresponsive to the output of said receiver prior to the reception of the reflected pulse from the opposite surface of the material, said indiclting means including a condenser, means for normally charging the condenser, means for discharging the condenser linearly, means actuated by the reflections received from a defect during the interval that the indicator is responsive for commencing discharging of the condenser, and means for continuing the discharge of the condenser until the indicating means is rendered ineffective.

7. In a device for testing material, means for generating a pulse, means for transmitting the pulse into the material through an entering surface thereof and for receiving reflections, a receiver actuated by said transmitting and receiving means, indicating meansactuated by the output from'said receiver, a two-part control for rendering said indicating means effective and ineffective, means for rendering one of said parts ineffective until the generation of the pulse ends, means for again rendering said part ineffective prior to the reception of the reflected pulse from the opposite surface of the material, means whereby rendering said first part eifective and ineffective renders said second part effective and ineffective, and means for actuating said second part by the output from said receiver.

RALPH B. DE LANO,-Js.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,009,459 Turner July 30, 1935 2,012,837 Tear Aug. 27, 1935 2,346,093 Tolson Apr. 4, 1944 2,415,119 Wellenstein Feb. 4, 1947 

